The present invention relates to a new and improved method of bleaching raw pulp used for manufacturing paper.
In its more particular aspects the present invention specifically relates to a new and improved method of bleaching raw pulp or fiber stock suspension prepared from waste paper and used for manufacturing paper. Such method may encompass oxidative, for example, peroxide bleaching as well as reductive bleaching using, for example, dithionite as the bleaching agent.
Both of the aforementioned bleaching methods at present usually constitute long-lasting, costly processes in terms of thermal energy. For carrying out such conventional processes, it is necessary to construct expensive structures such as, for instance, high bleaching towers to serve as reaction vessels, because the bleaching process requires a reaction time in the range of one to two hours.
According to one known method presently in use, dithionite is utilized as the reductive bleaching agent. A low-density pulp suspension having a consistency of 5% by weight is heated to 60.degree. C. and 1 to 2% of dithionite is admixed to this material in a double shaft mixer. This mixture is transferred into a bleaching tower through which the mixture is upwardly passed during more than 60 minutes for bleaching purposes.
Another known method using dithionite as the bleaching agent is the so-called MC method, i.e. medium consistency method which operates at a pulp or fiber consistency of 15% by weight. Here again, 1 to 2% of dithionite is admixed to the material. The mixer is a mixer pump, the so-called MC pump, which rapidly and satisfactorily admixes the bleaching agent. However, when using this method, air venting must be ensured and the MC pump must be set up such that the air is vented during the mixing process. The reductive bleaching process must be carried out in the absence of air because the bleaching agent is decomposed in the presence of oxygen. The material mixed with the bleaching agent has a temperature of approximately 60.degree. C. and, also in the process, the bleaching duration is approximately 60 minutes. Due to the higher consistency, a somewhat smaller bleaching tower can be used as compared to the aforementioned method utilizing low-density pulp.
With regard to oxidative bleaching, for example, by adding peroxide, such method has already been carried out at consistencies in the range of 15 to 30% pulp. The chemicals were admixed to the pulp in a low-speed disperser.
Reductive dithionite bleaching with pulp consistencies above 15% seems not to have been realized in practice. U.S. Pat. No. 2,963,395 proposes using pulp consistencies in the range of 20 to 50% and temperatures in the range of 38.degree. C. to 110.degree. C. for dithionite bleaching. However, the air must be removed to a large extent and the bleaching process must be carried out in an air-tight reaction vessel.
Thus, it was known in the case of high consistency, high temperature reductive bleaching using dithionite as a bleaching agent that it was necessary to remove and keep away the air from the bleaching system. This has been found difficult despite the use of complicated means and the expenditure resulting therefrom exceeded the expected savings.